Optical sensors for determining the moisture content of the soil in an irrigation system are well known. They usually take the form of a prism or similar structure, in which a light beam projected into the prism is internally reflected toward a photosensor such as a photodiode or phototransistor. (The term “light” in this application is meant to include infrared radiation). The amount of light received by the photosensor depends on the amount of moisture present at the surfaces of the prism. This moisture changes the optical characteristics of the prism surface and thereby causes a portion of the beam to be refracted outwardly of the prism, instead of being reflected inwardly toward the light sensor. The amount of refraction, and thus the amount of light received by the photosensor, translates into a measurement of the wetness of the soil.
It has previously been proposed in Benoit et al. U.S. Pat. No. 4,422,714 to use a transparent half-ellipsoid body as a level sensor in a container of mineral oil. In that patent, a fiber-optic light guide conveying substantially collimated light from a light source to the ellipsoid's surface is terminated at one of the foci of the ellipsoid, while a second light guide conveying light to a photosensor receives similarly collimated reflected light at the other focus of the ellipsoid. If all or part of the convex surface of Benoit's body is immersed in mineral oil, the resulting change in the index of refraction at the body-oil interface causes the light received by the photosensor to indicate not only the presence of a critical oil level but also whether it is rising or falling.
The above-described prior art construction is not, however, practical for soil moisture sensors because the presence of particulates in soil requires using the maximum available surface area of the ellipsoidal body as a reflection surface, so as to average the moisture effects over as large a surface of the sensor body as possible. This in turn requires a wide-angle light source and a wide-angle photosensor at the foci of the ellipsoid. One solution to this problem is shown in my copending application Ser. No. 11/214,100, filed on Aug. 29, 2005 and entitled Optical Moisture Sensor the contents of which are hereby incorporated by reference. That application discloses a cylindrical sensor with an interior refracting surface that causes divergent light rays to be refracted into parallelism so as to make optimum use of the cylindrical soil-contacting surface of the sensor.
A disadvantage of the sensor shown in the above-cited copending application in cold and moist environments is the fact that an air space needs to exist between the light source or photosensor and the internal refracting surface. In a cold environment, condensation can occur in that air space, and in a very moist environment, moisture can migrate through the sensor material. In either event, these conditions may adversely affect the parallelism of the internally refracted rays and may require special manufacturing precautions.
The aforesaid disadvantage can be overcome by mounting a wide-angle light source and photosensor in direct contact with a transparent ellipsoidal body. This does, however, cause several other problems. For one, a substantial portion of the light travels directly through the transparent body from the light source to the photosensor without being reflected by any body-air or body-water interface. Consequently, the sensitivity of such a sensor is substantially compromised.
Another problem arises in the manufacture of moisture sensors of the type described due to the fact that the light source and photosensor must be maintained in exact alignment with the foci of the ellipsoid during manufacture. This is necessary in order to produce consistent readings among mass-produced sensors. Also, the difference in coefficients of expansion between the body material and the circuit board on which the sensor's optical and electronic components are typically mounted can cause minute cracks adjacent the board into which moisture can migrate. It is therefore necessary to so encapsulate the light source, photosensor and associated electronics in the ellipsoidal body that moisture cannot cause any discontinuities between them and the body.